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Determining precise locations
- Learning outcomes (objectives)....................................................................... Table of content
- 2 Introduction............................................................................................................
- 2.1 Basic Principles of GPS......................................................................................
- 2.2 types of GPS receivers......................................................................................
- 2.3 parts and uses of GPS device...........................................................................
- 2.4 GPS segments.................................................................................................
- 2.4.1. Space Segment.......................................................................................
- 2.4.2. Control Segment.....................................................................................
- 2.4.3 User Segment..........................................................................................
- 3 Preparing for GPS data collection.........................................................................
- 3.1 Developing data collection plan.....................................................................
- 3.2 Errors in GPS...............................................................................................
- 4 Multi Path Error.................................................................................................
- 4.1 Atmospheric Effect......................................................................................
- 4.2 Ionospheric Delay:......................................................................................
- 5.1 GPS point positioning.....................................................................................
- 5.2 GPS Relative Positioning.................................................................................
- 5.3 Static GPS technique......................................................................................
5.4 Operating GPS Device..................................................................................... 31
5.5 collecting GPS data..................................................................................... 32
6 Processing GPS data............................................................................................. 33
6.1 Finalizing processed data............................................................................... 33
1. Learning outcomes (objectives) At the end of the module the learner will be able to: Plan and prepare Prepare for GPS data collection. Collect GPS data Finalize the collection process 2. Introduction This lesson covers the knowledge, skills and attitudes required to collect data using basic global positioning system (GPS) equipment and to be able to use suitable software to communicate with a GPS receiver. HANDOUT-REFERENCE TO MODULE Occupational Title: Surveyor Qualification Level: Level III TVET-Programme Title: Surveying Learning Module Tille : Determining precise locations
R R R Receiver Position R1 R2 R Receiver Position GPS consists, normally of constellation of 24 – operational satellites. This constellation, known as Initial Operational Capability (IOC) was completed in July 1993. The official IOC announcement, however, was made on December 1993. To ensure continuous worldwide coverage, GPS satellites are arranged so that four satellites are placed in each of six orbital planes. With this constellation geometry, four to ten GPS satellites will be visible anywhere in the world, if the elevation angle of 10^0 is considered. Only four satellites are needed to provide the positioning or location information in the form of precise estimation of position, velocity and time (PVT) by using a GPS receiver.
2.1 Basic Principles of GPS......................................................................................
GPS is based on the principle of resection i.e. Receiver’s position (x,y,z) can be determined given the distances from objects whose positions are known. Analytically, from the known positions of three objects and their distance from unknown position, we get three-distance equation involving three unknown corresponding to the unknown position of the receiver. These three equations can be solved for three unknowns (x,y,z). Fig: 1 Fig: 2 Theoretically, only three distances to three simultaneously tracked satellites are needed. R 1 , R 2 and R 3 in Fig: 1 In this case the Receiver would be located at the intersection of three spheres – each has a radius of one receiver-satellite distance. Fig: 2 Fourth satellite is required to account for Receiver clock offset.
Then the intersection of three distances from satellite to the receiver would be the position of receiver. Satellites broadcast their position as a function of time in WGS-84 Coordinates system. Therefore, the position of receiver is determined in WGS-84 coordinate system, which later converted into local coordinate system.
2.2 types of GPS receivers......................................................................................
Basically, there are two types of GPS receivers Hand held GPS receiver Fixed GPS receiver Hand held GPS receiver: uses point positioning to determine X,Y,Z coordinate of a point relatively low accuracy is required. It usually used for low accuracy navigation. Fig 3 Fixed GPS receiver: uses either point or relative poisoning. in Engineering application used for high accuracy applications such as surveying and mapping, GIS and precise navigation. 2.3 parts and uses of GPS device
A replica of C/A or P – code is generated by receiver is correlated with IF Signal. At this point the pseudo range is measured which is the time shift required to align the internally generated code with the IF signal, multiplied by the speed of light. A receiver generates another replica of the carrier. That carrier is correlated with IF signal and shift in phase can be measured. D. Channels The antenna itself does not sort the information gathered. The signals from different satellites enter the receiver simultaneously In the Channels of the RF section the signals are identified and differentiated from one another. It is a combination of hardware & of software to separate one signal from other At any given time- only one frequency from one satellite can be on one channel at a time A receiver can have 3 – 40 physical channels. 12-channel parallel receiver is common. E. Microprocessor – Processed signals are displayed on display unit and or at the same time recorded at storing device using micro- Processor. F. Power System – the entire system is attached to Battery 2.4 GPS segments GPS consists of three distinct segments:
- The space segments - Satellites orbiting the earth
- The control segments - stationed positioned on the earth's equator to control the satellites.
- The User segment - Anybody that receives and uses the GPS signal.
2.4.1. Space Segment.......................................................................................
The space segment is designed to consist of 24 satellites orbiting the earth at approximately height 20,200 km in every 12 hours. The satellites are arrayed in 6 orbital plane inclined 55^0 to the equator. The space segment is so designed that there will be a minimum of four satellites visible above a 15^0 cut - off angle at any point of the earth's surface at any time.
GPS Satellites Constellation Four satellites are the minimum that must be visible for most applications. Experience shows that there are usually at least 5 satellites visible for most of the time and quite often there are 6 or 7 satellites visible. Each GPS satellite has four very accurate and precise atomic clocks ( Rubidium and Cesium standards ) and has a microprocessor on board for limited self-monitoring and data processing. The satellites are equipped with thrusts, which can be used to maintain or modify their orbits. The clocks operate at a fundamental frequency of 10.23 MHZ. This is used to generate signals that are broadcast from the satellites. The satellites broadcast two carrier waves constantly. These carrier waves are in the L - Band (used for radio), and travel to the earth at the speed of light. These carrier waves are derived from the fundamental frequency, generated by a very precise atomic clock. The GPS satellites essentially provide a platform for radio transceivers atomic clocks, computers and various ancillary equipments used to operate the system. The electronic equipments allow the measurement of pseudo range (R) from receiver to satellite, and each satellite broadcast a message, which allows the user to determine the spatial position of the satellite at any instant. With these data, the users are able to determine their position by resection.
MONITOR STATION (MS) MASTER CONTROL STATION (MCS) GROUND ANTENNA (GA) Satellite
Receiver MASTER CONTROL STATION (MCS) - Prediction of ephemerides and clock behavior MONITOR STATION (MS) - Control of ephemerides and satellite clocks. GROUND ANTENNA (GA) - Uplink of Navigation Message to Satellites The main operational tasks of the control systems are : Tracking of the satellite for the orbit and clock determination and prediction modeling. Time synchronization of the satellites Upload of the data message to the satellites.
2.4.3 User Segment..........................................................................................
The user segment comprises of anyone using GPS receiver to receive GPS signals and determine their position and /or their time. Typical applications within the user segment are Land Navigation for vehicle location, surveying, marine navigation, areal navigation, machine control etc. GPS receivers vary greatly in size and complexity. The typical receiver is composed of an antenna and preamplifier, radio signal microprocessor, control and display device, data recording unit and power supply. The GPS receiver decodes the timing of signals from the visible satellites and calculates their distances. This is used to compute its own latitude, longitude, elevation and time. This is a continuous process and generally the position is updated on a second by second basis.
Rapid Static survey is suited for short baselines, if centimeter level accuracy is desired. For high accuracy over longer distance or in case of weak satellite geometry, it may be wise to use conventional Static GPS technique. The cost of GPS positioning is closely tied to the technique used, which in turn is chiefly a product of the accuracy requirements. Two major reasons for cost variations with techniques are the time on the site requirement and the cost of the required receiver. Generally, the shorter the time required on the site, the lower the survey cost. Selection of Receiver Type: Receiver selection can be a complex process due to large number of GPS receiver is now available and their wide range of capabilities and intend applications. To assist in receiver selection, Technical Specifications are considered based on following. a) Accuracy b) Single Frequency or Dual Frequency c) Code Measurements d) Data Processing capability e) Storage capacity f) No. of channels g) Compatibility with other related software etc. It is suggested that all GPS receivers used together for relative positioning be of the same make to avoid problems which often results from mixing receiver types such as Biases, Complexities in data processing and data rate incompatibilities. The receiver used must be capable of collecting the measurements needed for desired positioning techniques. E.g. for both single point positioning and relative positioning For Kinematic, Rapid Static and Conventional Static GPS surveys, code and carrier measurements receivers are required. For short Base line using conventional technique, single frequency receivers are sufficient.
For conventional static GPS survey over longer baseline where high accuracies are desirable, dual frequency receivers are useful since they permit corrections of most of the Ionospheric errors. For Rapid Static surveys, dual frequency receivers are strongly recommended. Validations, Reconnaissance, Survey Design and Preparation Validation One very important step in assessing receiver type as well as methodology and processing is the validation process. In the planning phase, the procedure and equipment to be used, from data collection to the final product should be tested to ensure that they reliably satisfy the desired accuracy requirements. This testing is referred to as validation process. Three main components are tested in validation process i) The positioning technique chosen ii) The equipment to be used and iii) The processing method adopted (Post or Real time) The positioning method chosen should be checked to ensure it would reliably satisfy the user requirements. The equipment used, processing of data technique adopted varies greatly in complexity and compatibility and hence it is important to test and evaluate them. The validation process also has the benefits of enabling users to identify and solve problems before commencing the project. Validation test should be carried out using points with known coordinates to an accuracy superior to that desired for the project. Reconnaissance: Another important step, which gives feedback in the planning process, is field reconnaissance. Reconnaissance consists of checking the field project sites for their suitability of GPS survey, availability of control points and logistic requirements. Through field reconnaissance obstruction and interference may be identified and avoided by alternative site selection or through establishment of eccentric stations.
3 4 5 6 7 Path of Rover Receiver
Figure: Radial Network Configuration Direct connection is made between each point and the control point rather than between adjacent points. Preparation: Some other important aspects of preparation of GPS survey are: - i) To determine and decide the best window (s) to be used to collect GPS data based on satellite availability and geometry. ii) To decide the optimal number of GPS receivers and personnel for the project and make the necessary arrangements. iii) To plot the survey design, taking into account of control requirements, network configuration, travel time between sites, satellite window and logistical constraints. iv) To establish the unique numbering or naming system to clearly identify all sites with their related computer files, positional information and other associated attributes. v) To arrange for transportation between sites (car, helicopter, boat or foot) vi) To organize all required equipments
GPS Errors and Biases vii) Finally, a set on condensed customized instructions be developed, specifying how exactly the receiver should be used in the field.
3.2 Errors in GPS...............................................................................................
GPS Pseudo Range and Carrier Phase measurements are both affected by several types of random errors and biases. These errors are classified as: - a) Errors originated at Satellites b) Errors originated at Receivers c) Errors due to signal propagation through Atmosphere (Atmospheric Effect) d) Geometric Location of GPS satellites also affects the accuracy (DOP) Errors Originated at Satellite: a) Ephemeris Errors or Orbital errors b) Satellite Clock Errors c) Selective Availability (SA) Errors Originated at Receivers: a) Receiver Clock Errors b) Multipath Error Errors due to Propagation of Signalthrough Atmosphere a) Ionospheric Delay b) Tropospheric Delay Errors due to Satellite Geometrical Location in the sky DOP, PDOP, HDOP, VDOP, TDOP, GDOP Ephemeris Errors or Orbital errors: Accuracy our computed position depends upon how accurately we know the location of the satellites Orbit of satellites are monitored continuously from monitoring stations and their predicted orbital information based on several GPS observations is uploaded to satellite which in turn is broadcasted
Errors in the satellite, orbital error, atmospheric effect, receiver clock error and multipath amount to less than 10 m of range error which under GDOP results error of 20 m. US Department of Defense (DOD) has determined to provide this level of precision to general public use DOD introduced man-made errors to degrade the positional accuracy of 100 m. This intentional degradation is called Selective Availability (SA) and is implemented by dithering the satellite clocks and reporting the orbit of satellite inaccurately. Receiver Clock Error Similar to satellite clock error, any error in receiver clock causes inaccuracy in distance measurement A typical clock has a drift of 1000 ns every second. Receiver correct their clock every second and adjust the receiver time to the accuracy of GPS clock Also, fourth unknown is used to correct receiver clock error when measurement to all the satellites taken at same time For this, we put receiver at a precisely known location, and then we need to track only one satellite to continuously calculate the receiver clock errors and adjust it. (as other three unknowns are known for that location)
4 Multi Path Error.................................................................................................
This is major source of error for both carrier and pseudo range measurements This occurs when GPS signal arriving at receiver antenna through different paths Therefore, the signals are direct line of sight signal and reflected signals from the object surrounding the receiver antenna The direct line of sight signal is distorted through the interference with the reflected signal Interference causes an uncertainty about the true signal arrive time much same way has eco of sound, resulting inaccuracy in range measurements
There are two way to reduce the effect of multipath:- i) Signal Processing Technique – where data is analyzed to separate the direct signal from indirect signal. It is applicable for reflected signal coming from the object having distance more than 10 m from the receiver antenna. If it is less than 10 m then the two signals cannot be differentiated. ii) By The Use of Choke Ring Antenna - it is a device that has several concentric metal hoops, which rejects the signal that hits the bottom side of the antenna an allows only those signals to hit the antenna coming from direct line of sight.
4.1 Atmospheric Effect......................................................................................
Signal travel time is measured to calculate the distance of satellite from the receiver which is multiplied with speed of light The speed of light varies due to atmospheric conditions.
4.2 Ionospheric Delay:......................................................................................
At the upper most part of the atmosphere, Ultra Violet (UV) and X-rays from sun interacts with gaseous molecules and atoms. These interaction results in gas ionization and that is why this region is called Ionosphere It extends from altitude of approximately 50 km to about 1000 km or even more Ionosphere is dispersive medium; it bends the GPS signals and changes its speed. The electron density varies with time and location, differencing the GPS observations over relatively short distance and different time removes the Ionospheric effect significantly. Tropospheric Delay: The layer of atmosphere up to 50 km is called troposphere. This contains water vapor as main components It slows the GPS signals therefore Only way is to remove Tropospheric delay is by measuring water vapor contents along with temperature and pressure and applying a mathematical model to compute the Tropospheric delay. Satellite Geometry Measures – Dilution of Precision (DOP) The various types of errors and biases discussed earlier directly affect the accuracy of the computed GPS position
